Conventionally, a seat slide locking apparatus is known which comprises a plurality of open engaging sections that are provided in a lengthwise direction of one of an elongate-shaped lower rail fixed to a vehicle body and an elongate-shaped upper rail fixed to a seat, and a locking piece which is held on the other of the lower rail and the upper rail in such a manner that it passes into a holding hole and can enter into and exit from the open engaging sections. One example of this seat slide locking apparatus is that disclosed in Japanese Patent Application Publication No. 2010-100169, for instance.
In the seat slide locking apparatus disclosed in Japanese Patent Application Publication No. 2010-100169, five locking pieces 111 formed to the same width W5 and thickness t5 are held on an upper rail 100 by holding members 110, as shown in FIG. 18 and FIG. 19. In this state, the five locking pieces 111 are inserted respectively in a vertically movable fashion into a first holding hole 100a provided in the upper rail 100 and a second holding hole 110a provided in the holding member 110. When the upper rail 100 is moved with respect to the lower rail 101, the locking pieces 111 enter into and engage with locking holes 101a that form the open engaging sections provided in the lower rail 101, due to the biasing force of springs, thereby locking the upper rail 100 immovably with respect to the lower rail 101.
Furthermore, in a seat slide locking apparatus of this kind, the first holding hole 100a and the second holding hole 110a described above are, in many cases, formed in a quadrilateral shape of a size into which the five locking pieces 111 can enter simultaneously as shown in FIG. 19 (in FIG. 19, the second holding hole 110a is shown).